Plaster, as a terminology generally accepted in the art, corresponds to partially dehydrated gypsum as a result of a calcination process performed under dry conditions. Gypsum (CaSO4.2 H2O), a calcium-sulphate dihydrate (DH) is thermally treated to remove part of the combined water and turn into metastable hemihydrate (HH; (CaSO4.1/2 H2O)) and dehydrated anhydrite (AIII or AII; (CaSO4 ε.H2O)) forms. Whether HH, AIII and AII is formed depends on the extent of the calcination temperature and conditions, e.g. vapour pressure.
While the gypsum becomes dehydrated, two mains types of hemihydrate are produced. The most commonly produced calcium sulphate hemihydrate is the “β-HH”, which results from the calcination of ground gypsum under normal atmospheric conditions. The other common type is called “α-HH” and results from the calcination of gypsum under hydrothermal conditions.
One of the major differences between the α- and β-forms of hemihydrate is the amount of water required to be admixed to give a pourable slurry (water demand), which is much higher for the β-hemihydrate than for the alpha hemihydrate. This is related to the different physical structures of the two forms in terms of porosity and crystallinity.
It is well known that HH-plasters harden to a solid matter if gauged with water, the reason being a substantial difference in the solubility between HH and DH. The phenomenon is called setting during which the recrystallisation starts and progresses after an initial germination period.
Due to its ability to build up new crystalline structure out of aqueous slurry, plasters are useful as a binder in the production of pre-fabricated gypsum elements such as plasterboards, gypsum fibre board and gypsum blocks. Generally, the β plasters are best suited for light weight fast setting products whereas the alpha plasters may find uses in for higher strength elements of construction.
In most cases, extra water (the water demand) is needed to achieve free flowable slurry which can be handled in the manufacture of the gypsum elements. However, the additional water must necessarily be removed in a final drying step which is very energy intensive and expensive. It must thus be reduced as much as possible.
The rheological properties of aqueous hemihydrates are dependent on the surface chemistry and the particle size and shape of the hemihydrate particles after mixing with water. This is particularly true when β-hemihydrate is involved, since the water demand is higher for this type of plaster.
Up until then, the calcined plasters were subjected to a forced ageing step in order to stabilize the crystalline structure and reduce the final water demand without impairing the mechanical properties of the product. Indeed, due to the rough thermal treatment the physical microstructure of β-HH is stressed and quite unstable. Thus one observes that, in contact with liquid water, a β-hemihydrate will partially disintegrate into very small particles. When absorbing humidity, the stress is lowered and the disintegration phenomenon fades. This treatment of the calcined β-HH is called “ageing”. This term does not however refer to “aridisation” which is essentially calcining in the presence of deliquescent substances.
The conventional stabilization processes as reported in EP 1 547 984 usually consist in a) providing a heated HH plaster at temperatures over 100° C., b) feeding the hot plaster in a moistening device, c) injecting water and/or steam, d) maintaining the atmosphere of the moistening conditions in order to cure the product at high temperatures f) feeding the moistened and cured blend into a drying device g) drying said moistened and cured blend and optionally h) grinding the dried product.
It is generally admitted that re-adsorption of water is the main promoter of ageing and several processes of the prior art such as reported in U.S. Pat. No. 1,713,879 involve the addition of water and/or steam to the calcined plaster.
In U.S. Pat. No. 3,415,910 the ageing process consists in quenching hot calcined hemihydrates with water whilst maintaining a temperature high enough to avoid the formation of dihydrate gypsum (DH) and performing a subsequent heating above 102° C. In GB 1233436, the treatment temperature could be as low as room temperature.
The European patent application EP 2 025 652 discloses a process for the preparation of stabilized anhydrite III to prevent its transformation into one of the hemihydrated forms. The envisaged solution consists in drying particles of natural hemihydrate at a temperature between 100 and 700° C. to provide anhydrite III in which additives such as water, diluted lime hydroxide, diluted cement, fluidizers or retardants are added by vaporization.
The PCT application WO 2008/115929 contemplates the addition of crystal modifiers at different steps of the calcination process to control the set of time of alpha hemihydrate plasters.
As regards the known ageing treatments of β-hemihydrates, ageing process is known from the patent application US 2008/0148998 which reports the post treatment of calcined β-hemihydrate using steam at a pressure above atmospheric pressure.
The patent application U.S. Pat. No. 4,360,386 disclosed the addition of a gypsum solubilizing agent into the aqueous wetting solution to shorten the time required for the ageing process.
Given the difficulties to improve efficiently the operating conditions, US 2008/0135072 and EP 1 547 984 show that the efforts of development were focused on the improvement of the apparatus implementing the stabilization process.
Nevertheless, in all the existing methods, the aged plaster still has the main drawback of developing strength on setting and unpredictable setting properties shortly after its treatment. Therefore, these plasters need to be dried if they are no used immediately which increases the difficulty of controlling the process, storage conditions, limits the production capability of the plant and requires additional equipment to dry the remaining plaster. In addition, the manufacturing facilities are most often limited in terms of capacities of production by the size and capacity of the dryers.
Therefore, there is a long felt need to provide an improved process which allows stabilizing the β-hemihydrate plasters durably.